Twilight settles on Beards Creek, one of a dozen creeks that grace Maryland's South River, now threatened by dvelopment. Photograph by Skip Brown.

As night falls on Beards Creek, red-winged blackbirds drop out of the twilight sky like black dice thrown by an invisible hand. They don't fly as much as fall fast, then with a flutter of wings easily break their speed and settle on the tips of marsh grass. The grass blades bob with the weight of the blackbirds in graceful silence.

Silence is a rare gift as it drops over the marsh that forms the headwaters of Beards Creek. Only a decade or two ago, the silence was as deep as the silt that fills the marsh. But each year has brought more people, more cars and trucks, and more noise from two busy highways beyond the trees. There Route 214 takes traffic east and west, and nearby Route 2 takes it north and south.

When the tide is high enough, the creek can carry a canoe on its thin watery trail through the marsh grass until the ground rises into thick woods. Here the stream deepens beneath a fallen tree and continues on against a gentle flow, right into the face of the forest. If the tide is high enough, you can push your canoe over the clutter of sticks and branches the beavers have piled to stem the flow, and drift right into the shallow flats of the beaver pond.

Though these beaver-tended woods seem inviolate here, the truth is that if the narrowing stream could carry your canoe only a few hundred yards farther, you would paddle right into Route 214. There inexorable lines of cars, trucks, and motorcycles make their way from the Bay to the Washington beltway and back. In the language of the local landscape, this intersection of stream and highway is a critical one.

Here the stream shrinks to pass through a culvert pipe beneath the road. The culvert channels the stream and alters its ability to rise and spread during heavy rainfall, changing its natural floodplain. The road, too, can be cut off during storm events, when the creek overcomes the culvert pipe and rises across Route 214. On those days, commuters returning from Washington encounter flares burning on the blacktop and a big orange sign that says, "Road Closed." Then they have to turn right and head the long way around the creek through the woods, until they reach Route 2 and turn north to find 214 again.

Except for these flood events, most motorists probably don't think about the stream or have any idea that they are driving over Beards Creek — the same creek where they may fish or water ski or sail or swim. One of those green road signs would be good here, to let commuters know that they are driving over Beards Creek. But when asked about installing such a sign, the State Highway Administration answered no, saying that this was "nothing but a pipe crossing."

Call it the psychology of pipes. According to Dennis Whigham of the Smithsonian Environmental Research Center, creeks and streams often confront this clogging of their arteries. Pipes constrict and channelize stream flow, causing erosion, downstream siltation and habitat destruction. Much better are bridges on raised pylons that allow the water to spread over natural floodplains. A bridge like that here would not only help the stream, it would cure the flooding problem.

In so many places we are changing the way water flows to the Chesapeake Bay. As forests and fields fall to roads and parking lots, streams become ditches. Gone are trees and long meanders that slow runoff. Unimpeded, sediment rushes toward the Bay and its rivers.

Tonight the tide is down, and I cannot canoe past the beaver dam. The old wooden paddle drips as it rests across the gunnels, and the canoe hardly moves, waiting like a patient horse for the time to head home.

I have seen fox and deer and raccoon and possum and beaver in this marsh. I have seen a wild turkey emerge on 214, its eyes wild as it ran like a bewildered old man through the baffling traffic until it found the safety of the underbrush.

This evening, as dusk comes, a winged shadow glides to the bare top of the only large tree in the center of the marsh. Barred owls love these lowlands and marshlands at the edge of forests. Its silhouette looms in the branches, and enough light remains for me to see its tawny back. I lift the binoculars just as he swivels his head. Two severe eyes, the size of silver dollars, fix me like crosshairs. The owl needs no telescope to look right through me.

When I lower the binoculars the bird seems farther off, high in his perch, regal, as though in command. Then the huge wings spread, and like a hang glider the owl pushes from his branch and soars down toward the dim marsh. At the same moment another owl appears from the left, rising to meet him. The two owls tangle and swirl, talon-to-talon, and pirouette down and out of sight. There is no sound.

I fear for the marsh. I fear for the creek.

The water so often now exudes a tan tint, a pale and sickly brown. I have seen this leathery color of runoff before. Several summers ago I took a kayak trip with my photographer friend Skip Brown on the Anacostia River. Skip was taking photographs for an article about the Anacostia for this magazine, and we paddled among rafts of plastic bottles and debris afloat in brown water.

So far Beards Creek still shines with another color, the luminous gray-green common to the Chesapeake — and even, on bright sunny days, a brilliant blue. But more and more runoff — often from construction — makes its way through a long gauntlet of pipes and spills its dark secrets into the creek. Then a shadow of sediment falls on underwater grasses, on oyster spat, on everything.